Introduction to Genetics

Punnet Squares - determining the outcomes

of a cross

Traits Which trait will the offspring have?

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F A T H E R S

vocabulary • Gene – location on a chromosome for a certain

trait.

• Allele - one form of a gene. • Cross – fertilization (mate). • Principle of Dominance – an organism with at

least one dominant allele will express that trait. The recessive trait will be expressed only when both alleles are recessive.

Law of Segregation

• During gamete formation, the alleles for each gene segregate from each other, so that each gamete carries only one allele for each gene

Genotype - allele combination determines Phenotype - observable characteristics (the way an organism looks)

Punnett Square

• A probability tool – used to predict the possible outcomes of a genetic cross

Types of genotypes

• If both alleles for the trait are the same, the genotype is homozygous – homozygous dominant (AA) – Homozygous recessive (aa)

• If the two alleles for the trait are different, the genotype is heterozygous – Heterozygous (Aa)

Some basic rules 1. Same letter for the same trait 2. Capital letter for dominant allele 3. Small letter for recessive allele 4. Dominant allele is shown first in each pair. Trait – eye color E = dominant allele e = recessive allele Homozygous dominant = EE Heterozygous = Ee Homozygous recessive = ee

Monohybrid cross • Definition: A monohybrid cross is a cross between

organisms that differ in one trait. Step 1, make a key

Flower color In flowers, Red (R) is dominant to white (r)

Possible phenotypes/genotypes Red

RR Rr

White rr

Monohybrid cross

• Step 2- write out parental genotypes

• Cross – Cross a white parent with a red parent

• Step 3 – Punnett Square

Homozygous parents RR x rr

R E D p a r e n t

W H I T E

p a r e n t

Genotypes and Phenotypes

Step 4 – record results Genotype

100% - Heterozygous (Rr) genotype Phenotype

100% - Red phenotype

Heterozygous parents Rr x Rr

R E D p a r e n t

R E D

p a r e n t

Genotypes and Phenotypes

25% - RR (homozygous dominant) genotype 50% - Rr (Heterozygous)genotype

25% - rr (homozygous recessive) genotype

75% - Red phenotype (RR + Rr) 25% - white phenotype (rr)

Dihybrid Cross

• Definition: A dihybrid cross is a cross between organisms that differ in two traits.

Dihybrid Cross

Possible gametes in the following genotypes: Genotypes Possible gamete combinations AABb AB,Ab Aabb Ab, ab aaBB aB aaBb aB,ab aabb ab AaBb AB,Ab,aB,ab

Dihybrid Cross

R – red

r – white

T – tall t – short

Dihybrid Cross RrTt x RrTt

Genotypes RRTT (1/16) – 6.25 % RRTt (2/16) – 12.5% RrTT (2/16) – 12.5% RrTt (4/16) – 25% RRtt (1/16) – 6.25% Rrtt (2/16) – 12.5% rrTT (1/16) – 6.25% rrTt (2/16) – 12.5% rrtt (1/16) – 6.25%

Phenotypes

RT red, tall (9/16) – 56.25%

Rt red, short (3/16) – 18.75%

rT white, tall (3/16) – 18.75%

rt white, short (1/16) – 6.25%

Understanding the coin toss - Two possible outcomes. - Only one result.

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Traits Which trait will the offspring have?

Two possible alleles – one from each parent. Only one goes to each gamete.

Dihybrid – two traits

Each parent has two coins

Simply working with two traits

Heterozygous Smooth and Yellow

Heterozygous

Smooth and

Yellow

Surface R = smooth r = rough Color Y = Yellow y = green

RrYy

RrYy

Dihybrid Cross RRTT x rrtt

RrTt R T R T R T R T

rt

rt

rt

rt

Dihybrid Cross

SSbb x ssBB

tail length (S or s) - short is dominant - long is recessive color of fur (B or b) - brown is dominant - white is recessive

Generations

Or . . . Work with each trait separately.

DOMINANT TRAITS RECESSIVE TRAITS

Eye coloring brown eyes grey, green, hazel, blue eyes

Vision

farsightedness normal vision normal vision normal vision

normal vision nearsightedness night blindness color blindness*

Hair

dark hair non-red hair curly hair full head of hair widow's peak

blonde, light, red hair red hair straight hair baldness* normal hairline

Facial features

dimples unattached earlobes freckles broad lips

no dimples attached earlobes no freckles thin lips

Appendages

extra digits fused digits short digits fingers lack 1 joint limb dwarfing clubbed thumb double-jointedness

normal number normal digits normal digits normal joints normal proportion normal thumb normal joints

Other

immunity to poison ivy normal pigmented skin normal blood clotting normal hearing normal hearing and speaking normal- no PKU

susceptibility to poison ivy albinism hemophilia* congenital deafness deaf mutism phenylketonuria (PKU)

• sex-linked characteristic